First ofcourse we have
single-<posttype>.php but I don't think that's what you want ( yay if it is )
To implement this, you would need a metabox, with a dropdown that saves the value to a custom field, which you would then pull from in
single-<posttype>.php and then use in an
include statement. single-.php would thus only output headers etc if no option was specified, else it would use
include to load the correct template
How you populate that combobox/dropdown is up to you. You may have 5 preset templates and hardcode the filenames, or you may decide to put in code that checks each files header like Wordpress does with pages.